Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid should carry cuttings from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that within the scope of the description and claims herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids”.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Brine-based drilling fluids, of course are a water-based mud (WBM) in which the aqueous component is brine. Oil-based muds (OBM) are the opposite or inverse. Solid particles are suspended in oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds can be either all-oil based or water-in-oil macroemulsions, which are also called invert emulsions. In oil-based mud the oil may consist of any oil that may include, but is not limited to, diesel, mineral oil, esters, or alpha-olefins. Solid particles, such as weighting agents, may also be suspended in WBMs, with the use of various viscosifying or suspending agents to help keep the solids in suspension.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. Because drilling fluids are called upon to perform a number of tasks simultaneously, this desirable balance is not always easy to achieve.
The density of aqueous drilling fluids may also be increased by dissolving salts therein. The density of the fluid is important because it must balance the density of the fluids in the formation and prevent them from entering the wellbore during drilling. Such salts include, but are not necessarily limited to, sodium chloride, magnesium chloride, calcium chloride, potassium chloride, sodium formate, potassium formate, cesium formate, sodium bromide, calcium bromide, zinc bromide, ammonium chloride, zinc chloride, calcium nitrate, potassium acetate, magnesium acetate and combinations thereof. Sometimes a water-based drilling fluid may be saturated or near-saturated with salts and a change will occur, such as a change in pH, fluid temperature, etc., and the salts will then settle and precipitate out. Such a phenomenon has at least two disadvantages: the salts will not be able to perform their stated function, and the solid salts may cause problems and difficulties downhole and may have to be removed in a separate operation.
Additionally, in situations where an operator is drilling through a formation containing salt, it is important that the drilling mud be near-saturation, at saturation or supersaturated with salts to inhibit or prevent the drilling mud from leaching salt from the formation during drilling. This is a particular concern when drilling through a salt dome, massive “sub-salt” beds typically found in offshore geological formations which many times are hundreds to thousands of feet in thickness and depth or similar formation. As used herein, the term “at least saturated with salt” includes the condition of saturated with salt and supersaturated with salt.
It is thus desirable that methods and compositions be devised to inhibit or prevent the settling out or precipitation of salts from an aqueous drilling fluid containing them.